The present invention relates to a method and apparatus for detecting a tracking error signal, capable of performing high-speed access to the inner and outer radii of an optical disk corresponding to a ROM disk, a writing once disk, or an erasable disk and, more particularly, to a method and apparatus for detecting a tracking error signal for a so-called separation type optical head in which an objective lens actuator portion which tracks variations in the axial direction (focusing direction) of the disk and the radial direction (tracking direction) of the disk is separated from an optical head main body.
In recent years, studies of an external storage apparatus having the high-speed accessibility of a magnetic disk used in a computer system and a large-capacity memory of an optical disk have rapidly advanced. An optical disk apparatus is expected as a next-generation disk apparatus because the optical disk apparatus has a high transfer rate, high-speed random access, a large-capacity memory, good preservation for a medium, and high durability effected by non-contact recording/reading for the medium.
In some optical disk apparatus, only an objective lens actuator is moved from the inner radius of a disk to the outer radius thereof, and the optical system of an optical head is fixed. This is to minimize the weight of a movable member and to increase a moving speed.
FIG. 3 shows a conventional separation type optical head. Referring to FIG. 3, a beam emitted from a laser 31 is collimated into a parallel beam by a collimator lens 32, and the parallel beam passes through a compound prism 36 obtained by integrating a wedge-shaped prism 33 and beam splitters 34 and 35, and a parallel beam emerges from a stationary optical system 37. The exit beam from the stationary optical system 37 is deflected 90.degree. by a 45.degree. mirror 39 incorporated in the lower portion of an objective lens actuator 38 moved from the inner radius of an optical disk (to be referred to as a disk hereinafter) 41 to the outer radius thereof, and reaches an objective lens 40.
The light beam focused by the objective lens 40 is reflected by the disk 41 and incident on the stationary optical system 37 in an order reverse to that of the above optical path, and a magneto-optical signal, a focusing error signal, and a tracking error signal are detected through the compound prism 36. More specifically, a light beam deflected by the beam splitter 35 of the compound prism 36 is detected through a polarization optical system (not shown). On the other hand, the light beam deflected 90.degree. by the beam splitter 34 of the compound prism 36 propagates straight through a condenser lens 42 and a half mirror 43 to reach a 2-division photosensor 44. The light beam input to the 2-division photosensor 44 is photoelectrically converted by two elements 45 and 46 constituting the 2-division photosensor 44, and a differential amplifier 47 differentially detects two signals from the elements 45 and 46 to obtain a push-pull tracking error signal.
A focusing error signal is detected by, e.g., a knife-edge method, from the light beam deflected 90.degree. by the half mirror 43. More specifically, one semicircular component of the circular light beam deflected by the half mirror 43 is cut, and the other semicircular component is input to a 2-division photosensor 49 arranged at the convergence point of the light beam. Two signals are output from the 2-division photosensor 49, and the two signals are differentially detected to obtain a focusing error signal.
In the separation type optical head in which the actuator is separated from the optical system main body, when the optical system is arranged at the outer radial portion of the disk 41, and the objective lens actuator 38 is moved to the inner radial portion of the disk 41, the distance between the disk 41 and the photosensor 44 for detecting a tracking error signal becomes maximum. At this time, when decentering of the disk 41, deformation of the disk, or inclination of the disk in a dynamic mode during rotation of the disk 41 occurs, the beam is moved to a portion shifted from the center of the 2-division photosensor 44 because the distance between the disk 41 and the 2-division photosensor 44 for detecting the tracking error signal is long. FIG. 4A shows this beam tracking operation for easy comprehension.
Referring to FIG. 4A, a designed optical axis is indicated by a solid line 50, and a beam reflected by the disk 41 is indicated by a broken line as a return optical axis 51. When the disk 41 is rotated, the disk 41 is mechanically deformed to be inclined by a dynamic resonance mode. Otherwise, when the disk 41 is arranged on a platter 52, the disk 41 is mechanically inclined. For this reason, the beam reflected by the disk 41 passes through the optical path of the return optical axis 51 indicated by the broken line extended from the disk 41. The reflected beam on the return optical axis 51 is transmitted through the beam splitter 35, and deflected 90.degree. by the beam splitter 34, and focused by the condenser lens 42. A tracking error detection beam reaches a position which is separated from the center of the photosensor on the 2-division photosensor 44. A tracking error signal is obtained by differential detection performed by the 2-division photosensor 44.
FIG. 4B shows a tracking error signal 53 obtained when the disk 41 is not inclined, and FIG. 4C shows a tracking error signal 54 having an actual offset variation and generated by the inclination of the disk 41. Referring to FIGS. 4A and 4B, when a tracking servo operation is performed by the tracking error signal 54 having the offset variation, a tracking operation is performed while the beam drifts between tracks, and the tracking operation becomes unstable. In addition, the envelope of a reproduced signal loses uniformity, and the signal cannot be stably reproduced.